The process by which T cells are activated, and the consequences of activation (e.g. new gene transcription, cytokine production, apoptosis), are being investigated by a variety of approaches: 1) studies on the consequences of losing CD45 (an abundant transmembrane tyrosine phosphatase) on activation of src family kinases (especially Lck and Fyn). We have found that these kinases are hyperactive in the absence of CD45 due to hyperphosphorylation of a conserved tyrosine in their catalytic domains. 2) the possible role of phosphatidylinositol-3 kinase (PI-3 kinase) in transducing activating signals via the T cell antigen receptor (TCR). Using both pharmacologic inhibitors of PI-3 kinase activity and dominant negative forms of the PI-3 kinase p85 subunit, we have demonstrated a critical role for this enzyme in IL-2 production. 3) signaling via Fas results in apoptosis. Based on other models of apoptosis, it has been hypothesized that Fas signaling is dependent upon activation of JNK (N-terminal Jun kinase). We have undertaken extensive analyses of JNK and AP-1 activation and have used dominant negative forms of JNK and AP-1, and have found that JNK is neither necessary nor sufficient for Fas-induced apoptosis. 4) activation-induced apoptosis of T cell hybridomas is mediated by upregulation of the ligand for Fas (FasL). We have cloned the FasL 5' regulatory region and are in the process of determining which transcription factors bind this area and which are involved in the regulation of FasL expression. 5) we have discovered that signaling via Fas results in the rapid downregulation of D3 cyclin, a protein required for cell cycle progression. The mechanism for this (transcriptional vs. post-transcriptional vs. post-translational) is being investigated. 6) we have found that corticosteroids prevent activation-induced apoptosis, and have hypothesized that this phenomenon regulates antigen-specific selection of thymocytes. Using transgenic mice that express antisense glucocorticoid receptor in immature thymocytes, or fetal thymic organ culture with drugs that inhibit corticosterone production, we have shown that locally-produced glucocorticoids do in fact regulate thymocyte development, and that this requires occupancy of the TCR (i.e. is antigen-specific).